1. Field of the Invention
The field of the present invention is vacuum boosters of the type in which a booster piston is contained in a booster shell for a longitudinal reciprocal movement and has a front surface facing a vacuum chamber to be connected to a vacuum pressure source, a diaphragm being superposed on a rear surface of the booster piston and having a rear surface facing a working chamber adapted to be selectively put into communication with the vacuum chamber or the atmosphere through a control valve which is connected to an input rod, the diaphragm having an outer peripheral bead coupled to the booster shell, a piston boss carried on a central portion of a rear wall of the booster shell for longitudinal reciprocal movement and provided with an annular step with which an inner peripheral bead of the diaphragm engages from the rear, and a stopper retained on the piston boss and formed in an annular fashion to abut against the inner peripheral bead from the rear.
2. Description of the Prior Art
Such vacuum booster is conventionally known, for example, from Japanese Utility Model Laid-open No. 114474/89.
In the above prior art vacuum booster, the stopper is formed by punching a metal sheet into a circular flat plate-like configuration so as to form, on its inner peripheral edge, a chrysanthemum-shaped fixture to engage the piston boss. The stopper is allowed to abut against a rear surface of the inner peripheral bead of the diaphragm in assembly. Accordingly, when a force is applied to the inner peripheral edge of the diaphragm in a direction away from the step of the piston boss due to deflection of a radially intermediate portion of the diaphragm toward the vacuum chamber forward of the booster piston during a boosting operation, the inner peripheral bead of the diaphragm is retained by the stopper and thereby prevented from being separated from the piston boss. However, if the booster piston is urged forwardly in a condition in which there is no vacuum pressure in the vacuum pressure source and no vacuum pressure is stored in the vacuum chamber, the working chamber is put out of communication with the vacuum chamber and into communication with the atmosphere by a control valve, so that the air in the vacuum chamber is compressed and intended to be discharged toward the vacuum pressure source. But the pressure in the vacuum chamber may be increased due to a resistance against such discharge. In this case, the diaphragm is deflected toward the working chamber about a portion thereof corresponding to the outer peripheral edge of the stopper. However, if the stopper is formed by punching a metal sheet as in the prior art, the outer peripheral edge thereof is not smooth, and if the rear surface of the diaphragm is urged strongly against the outer peripheral edge of the stopper as described above, such portion of the diaphragm may be damaged. Moreover, when the diaphragm is deflected toward the working chamber due to an increase in pressure in the vacuum chamber as described above, a force is applied to the inner peripheral bead of the diaphragm in the direction away from the step of the piston boss. Therefore, it is necessary for the stopper to have a rigidity sufficient to retain the inner peripheral bead on the step against a slip-out load acting on the stopper, and it is necessary for the stopper to engage the piston boss with a sufficient strength.